Already known is a three-way catalytic converter, by which three major toxic components (HC, CO and NO.sub.x) in the gas exhausted from an internal combustion engine can be eliminated. The operation of this known three-way catalytic converter is effectively attained when an atmosphere of the exhaust gas introduced into the converter is controlled near stoichiometric atmosphere in which excess air or excess fuel is, substantially, not left in the exhaust gas, i.e., excess air ratio .lambda. of the exhaust gas is kept near 1.0.
However, it is impossible to keep the .lambda. near 1.0 in a known internal combustion engine, especially, in a carburetor type internal combustion engine, therefore, effective operation of the three-way catalytic converter is not expected.